1. Field of the Invention
This invention relates to a sewing machine in general and more particularly to a control system for stopping a sewing machine needle at a predetermined position.
2. Description of the Prior Art
An electric or electronic sewing machine has an upper drive shaft driven by an electric motor and a stitching needle connected to the upper drive shaft. One reciprocation of the needle in a vertical direction is governed by one rotation of the upper drive shaft to produce a stitch on a fabric.
The needle should preferably be stopped at a predetermined position, typically at an uppermost dead point or in some case at a lowermost dead point, after a desired series of stitch patterns have been produced on the fabric, which will facilitate the next stitching operation. Several control systems have been proposed for controlling the needle stop position.
FIG. 5 shows an example of the prior art control systems disclosed in Japanese patent provisional publication No. (Sho) 54-127752. While the upper drive shaft is rotating at a relatively high rotational speed, a stop command signal becomes "H" level at a time (tc). The rotational speed will therefore decrease proportionally toward a predetermined value, for example 120 r.p.m. This rotational speed is determined such that the upper drive shaft may soon come into a standstill within one rotation. After the upper drive shaft rotational speed has decreased below 120 r.p.m., the electric motor is deenergized at the next rise of a stop phase signal, at a time (t.sub.1) in this example. Thus, the upper drive shaft rotational speed will further be decreased so that it stops at substantially a constant phase.
This prior art system, however, will not provide an accurate control. The inclination of decrease of the upper drive shaft rotational speed is varied with load fluctuation especially at a head portion of the sewing machine. If the sewing machine has a greater load, the rotational speed decreases more rapidly so that the needle stopping position would be offset from a predetermined position, as shown by dotted lines.
This defect could be eliminated by another prior art control system shown in FIG. 6 which is disclosed in Japanese patent provisional publication No. (Sho) 63-262080. In this prior art, load fluctuation is previously calculated to determine a corresponding pulse number of a pulse signal, which generates 360 pulses per rotation, for example. The electric motor is deenergized at a time (ts) which comes after counting the pulse number thus determined from a time (t.sub.1) when the upper drive shaft rotational speed has been lowered below a predetermined value or 140 r.p.m. in this example. Thus, it is expected that the upper drive shaft may always be stopped at a time (t.sub.2) to provide a constant needle stopping position. However, since the inclination of decrease of the upper drive shaft rotational speed is not directly determined, the actual needle stopping position would be varied due to a sudden load change. Furthermore, if the upper drive shaft rotational speed decreases as shown by double dotted lines, this prior art system discriminates that the rotational speed is still higher than the predetermined value, namely 140 r.p.m. at (t.sub.1) so that an additional one rotation will be required, although in this case the upper drive shaft can be stopped at a predetermined phase or position within one rotation.